The consumption of traditional energy resources has caused environmental pollution and drastic changes in global climate, which in turn indirectly leads to the largely increased demands of people for more energy sources. To solve the problem of worldwide energy shortage, all the countries in the world are devoted to the exploitation of alternate energy resources and expect the alternate energy resources can reduce people's demands for the traditional energy resources to thereby solve the worldwide energy crisis. Although solar radiation can be converted into electric energy through solar photovoltaic technology, and light energy from the sun is non-depletable, the currently available sunlight energy conversion efficiency is relatively low. Therefore, scientists make all efforts to work out an effective solution for improving the sunlight energy conversion efficiency.
In a solar photovoltaic system, a photovoltaic (PV) cell is used to convert the solar energy into electric energy for supplying to a load. Alternatively, a plurality of photovoltaic cells can be grouped into several groups. The photovoltaic cells in each group are connected in series and then, all the groups of serially connected photovoltaic cells are connected in parallel to form a photovoltaic cell module (PVCM). Through the connection of the photovoltaic cells in series and in parallel, the electric energy converted from the solar energy can have a predetermined voltage. The electric energy with the predetermined voltage can be supplied to various electric appliances via a voltage converter, or be stored in a battery. No matter the solar photovoltaic system comprises only one single photovoltaic cell or a photovoltaic cell module including a plurality of serially and parallelly connected photovoltaic cells, the generated power would change with the amount of solar radiation collected by the solar photovoltaic system. As a result, the output current and voltage of the solar photovoltaic system is unstable. Further, the voltage converter is designed based on a certain specific input voltage and current. The unstable voltage and current generated by the photovoltaic cell module input to the voltage converter at non-optimal settings in most of the time. Thus, the voltage converter has relatively low power conversion efficiency (PCE). That is, a large amount of solar energy is wasted.
Please refer to FIG. 1. In practical application, after a photovoltaic cell module 91 has generated electric energy, the generated electric energy is converted by a voltage converter (not shown) in a power management system 92 into a low voltage output 921 and a negative voltage output 922 for supplying to an applied circuit 94. Also, the generated electric energy is converted into a high voltage output 923 for supplying to an input/output (I/O) component 95. Furthermore, surplus electric energy is stored in a battery 93. When the electric energy converted from the solar radiation is insufficient, the power management system 92 would cause the electric energy stored in the battery 93 to convert into a suitable voltage for supplying to the applied circuit 94 and/or the I/O component 95.
There are prior art techniques for increasing the power conversion efficiency of the voltage converter. For example, in the thesis by Shao et al. entitled “A Micro Power Management System and Maximum Output Power Control for Solar Energy Harvesting Applications”, IEEE International Symposium on Circuits and System (ISCAS) 2007, pp. 298-303, New Orleans, La., USA, 27-30 May 2007, the output power of the voltage converter is tracked and optimized to increase the power conversion efficiency of the voltage converter. However, in implementing the micro power management system as suggested by Shao et al., it is necessary to cope with the input electric energy under various conditions in order to perform the power regulation. Accordingly, the voltage converter for this purpose would have complicated structure, large volume, high manufacturing cost, and can not be suitably applied to small-size electronic products, such as mobile phones, portable computers, and personal digital assistants (PDAs). Moreover, the complicated control circuit of the power management system would consume additional power and could not be suitably applied in the μW-level power management circuit. US Patent Publication No. 20070182362 discloses the use of a supercapacitor to store the electric energy therein and the stored electric energy is coordinately supplied to batteries or external loads. While the way suggested in this patent can be easily achieved, it is still difficult to reduce the cost for a supercapacitor with sufficient capacity. National Semiconductor Corporation, USA announced its SolarMagic™ technology on Jul. 17, 2009. According to the SolarMagic™ technology, the output power of every photovoltaic cell is monitored and optimized to adjust the current direction in the photovoltaic cell and accordingly increase an overall efficiency thereof. That is, the SolarMagic™ technology adopts the technique of control by source. However, since the optimizing strategy is complicated and requires an additional voltage conversion chip, it is very difficult for the power management system based on the SolarMagic™ technology to be configured as a system on chip (SoC).
Presently, the photovoltaic cell has increasingly widened applications, and can be applied to, for example, domestic solar water heaters, solar street lamps, and various types of consumer electronic products and portable electronic products. The demand for photovoltaic cell keeps increasing. Therefore, it is necessary to develop a power management system that has simple structure and effectively enables increased solar power conversion efficiency.